1. Field of the Invention
The present invention relates to a method and an apparatus for injection-molding a molten resin wherein any flaw such as warpage, shrinkage or the like does not occur in parts having a small thickness when a desired product of injection-molded resin is molded by injecting the molten resin into a cavity of an injection molding die.
2. Background Art
With respect to a conventional injection-molding method, to prevent not only reverse flow of a molten resin filled in a cavity of an injection-molding die, but also to reduce injection pressure of the molten resin to be injected into the cavity, and moreover, to compensate for contraction of the injection-molded product caused as the molten resin is cooled, pressure retaining is performed after the injection-molding die is fully filled with the molten resin. Since the pressure to be retained at that time differs from injection-molded product to injection-molded product, there arises an occasion wherein pressure retaining is controlled by way of multi-stages.
However, when the step of pressure retaining is performed during a conventional injection-molding method, immediately after a molten resin is injected into an injection-molding die, which in turn is fully filled with the molten resin, flaws such as warpage, shrinkage or the like sometimes locally occur in a product of injection-molded resin. This phenomenon can be prevented to some extent by modifying the present geometrical configuration of the injection-molded product or the present design of the cavity gate. However, in the case in an injection-molded product in which the geometrical configuration and gate design are restricted, the conventional injection-molding method does not include any means for preventing warpage and shrinkage from occurring an injection-molded product.